It is well known that particle accelerators are used to accelerate ions (protons and heavier ions) to very high velocities. At high velocities, a large number of such particles form what is called a “beam”, and this beam can be used for different purposes, for instance research, medical or industrial applications. Early accelerators' cost and size practically limited the utilisation thereof to research laboratories. Even today, the existing accelerators are often unpractical for many applications making use of ions.
Existing accelerators are of three kinds: cyclotrons, linacs and synchrotrons.
If the request is for ion beams of large mass-over-charge ratio and/or for the velocity range up to about 0.6 times that of light, conventional cyclotrons are less suited. Compactness, modularity, less complexity and as a result lower cost are the advantages of linacs with respect to synchrotrons.
The technology of radio frequency (RF) linacs is currently used for the acceleration of charged particles from an “ion source” to the desired energy. For ions (protons and heavier ions), the energy range covered by linacs is of several tens of kilo-electron-volts per nucleon (keV/u) to hundreds of million-electron-volts per nucleon (MeV/u), i.e. a velocity range from about 0.05 to about 0.9 times that of light. Several types of linacs, which are maximally efficient in a particular energy sub-range, have been developed. If a large range has to be covered, different linac structures, each optimally chosen in its frequency range, are serially disposed, with a consequent increased complexity and cost of the whole system.
All linac designs generally consist of evacuated cylindrical type metallic cavities or transmission lines. These structures are filled with electromagnetic energy by RF power generators. The beam passes through the longitudinal axis of the linac and encounters strong RF electric fields that can accelerate the charged particles, if the phase of the RF wave is appropriately synchronised with the arrival of the bunched beam.
To date, two kinds of structures have been used: travelling wave and standing wave structures. In travelling wave structures, the accelerator is a transmission line and behaves like a waveguide in which the electromagnetic waves travel along the whole length of the structure. Some power is delivered to the beam, some power is lost due to ohmic losses and the rest is dumped in a matched load. In standing wave structures, the accelerator is a resonant cavity inside which the injected electromagnetic waves establish a time-dependent standing wave pattern, periodic at the resonant frequency.
It is well known that a parameter commonly employed in this field is β=v/c, where v is the velocity of the particles and c is the velocity of light. Standing wave linacs are mainly used for particle velocities less than half the speed of light (low β linacs). Both standing wave and travelling wave linacs are used for higher velocities (medium β linacs), with the current trend in favour of the first solution. At v≈c, travelling wave linacs predominate (high β linacs). It is also known that deep cancer therapy with light ion beams requires β≦0.6, which is in the range of standing wave linacs.
Moreover, it is known that:                in the low velocity range (0.01≦β<0.1), the most commonly used linac structure is the Radio-Frequency Quadrupole (RFQ),        in the middle velocity range (0.1≦β≦0.4), the most used is the Drift Tube Linac (DTL) structure,        the Coupled Cavity Linac (CCL) structure is the standing wave structure most used in the high velocity range (0.4≦β<1).        
In standing wave linacs, the RF electric fields are applied inside evacuated resonant cavities to a linear array of electrodes. The spacing between the electrodes is arranged so that the field in an appropriate phase with respect to the beam arrival delivers “useful” power to the particles. The rest of the time, the field is shielded and does not act on the bunched beam. The spacing between successive electrodes also takes into account the increase in particle velocity, leading to longer structures for higher velocity beams.
The RF electric fields in these cavities result from the excitation of resonant electromagnetic cavity modes. Normally, the field pattern is contained in a cylindrical volume. In such a volume, two family modes can exist:                transverse magnetic modes (TM), also called E-modes, where a strong electric field component exists along the beam direction (or, in other words, the magnetic field is transversal to the beam direction),        transverse electric modes (TE), also called H-modes, where a strong magnetic field component exists along the beam direction (or, in other words, the electric field is transversal to the beam direction). In this latter family, the insertion of the electrodes modifies the field pattern from the just exposed configuration, in such a way that a strong electric field component is always directed along the beam direction, which is the useful direction.        
Experience in cavities development with both types of standing wave patterns has led to understand the different behaviour of cavities using E-modes or H-modes.
In E-mode families, the insertion of the electrodes does not affect very much the direction of the accelerating field, which is already directed along the beam direction.
On the contrary, in H-mode families, the insertion of the electrodes drastically re-directs the accelerating field along the beam axis. As a result, in H-mode cavities, the electric field is better concentrated close to the beam axis, where it is effectively needed. Therefore, H-mode structures are more efficient.
A parameter commonly used to measure the efficiency of the cavity with respect to power consumption is the “shunt impedance per unit length”. This parameter has the dimensions of a resistance per unit length and is independent on the field level and on particle velocity.
Generally speaking, H-mode cavities have quite large effective shunt impedance per unit length, decreasing when the particle velocity increases, while E-mode cavities have the opposite behaviour. Therefore H-mode cavities are more efficient at low velocity, while E-mode cavities are better at high velocity, the crossover usually being placed at around β≈0.4.
The longitudinal dimensions of the accelerating structure are linked to the length travelled by the particles in an RF period, also called the “particle wavelength” or βλ, where λ is the RF wavelength. Efficient acceleration occurs when the particles arrive at each accelerating gap with the appropriate RF phase. In an RF linac, two working modes are possible: 0-mode and π-mode. Considering the RF field at a given time, in 0-mode the on-axis accelerating field has the same module and sign at each accelerating gap, while in π-mode the electric field changes sign from one gap to the next. The current trend is in favour of the π-mode, since, for the same βλ the effective average field gradient is higher.
A more detailed description of the particle accelerators used to date can be found in the references at the end of this description, listed by publication date.
Finally, it must be pointed out that the field of application has a major impact on the choice between the existing types of proton and ion accelerators of different structural characteristics and functionalities:                in radiotherapy, the requirement is for extremely precise, very low intensity pencil beams of limited energy and small energy spread. Preferably, they have to be delivered by reasonably small and compact structures to be installed in the limited space available in a hospital environment, while        in the field of research, the requirement is often for high intensity and high-energy beams for experiments, for instance in high energy physics, or related to nuclear fission, fusion and many other applications.        
U.S. Pat. No. 5,382,914 discloses a linac for proton therapy, the structure of which is rather conventional and the DTL practically represents the well-known Alvarez structure. The 0-mode is used for acceleration in the DTL linac and the latter is considerably long.
U.S. Pat. No. 5,523,659 relates to a radio frequency focused DTL having a known Alvarez structure with modifications including RF focusing sections of the RFQ type. The mechanical construction including the electric focusing is complex. The resulting shunt impedance is low and the resulting coupling between longitudinal and transverse planes complicates the beam transport.
U.S. Pat. No. 5,113,141 discloses a four-fingers RFQ linac structure, which is a H-mode cavity structure, making the attempt to focus and accelerate at the same time low energy beams. The efficiency of this kind of focusing rapidly decreases as β increases. The resulting shunt impedance is low and the resulting coupling between longitudinal and transverse planes complicates the beam transport.
U.S. Pat. No. 4,906,896 relates to a disk and washer linac the structure of which makes use of E-modes. At low β the shunt impedance is low. The mechanical construction is complicated. The field stability is rather low since it is perturbed by RF resonances close to the working mode.